1. Field of the Invention
The present invention relates to a magnetron sputtering apparatus and, more particularly, to a magnetron sputtering apparatus adapted to prevent local erosion of the target, thereby prolonging the life of the target.
2. Related Art
A conventional magnetron sputtering apparatus, shown in FIG. 13 in a square form, comprises a target 1 made of a ferromagnetic material, an inner magnetic pole 2 mounted on the reverse side of the target 1, and an outer magnetic pole of opposite polarity and surrounding the inner magnetic pole 2. These members are positioned in a vacuum chamber. The target 1 is usually mounted on a backing plate made of a non-magnetic material such as copper. A coil 4 is also provided to apply an electric current to energize the magnetic poles 2, 3. Although the inner and outer magnetic poles 2 and 3 are usually made of an electromagnet, they may be of a permanent magnet.
In using the magnetron sputtering apparatus, an energizing current is applied to the coil 4 that provides the target 1 with a magnetic field in the direction as shown by the arrows in the drawing, and A.sub.r sputtering ions, for example, bombard the target 1, thus applying sputtered particles on an article (not shown) to be coated.
FIG. 14 shows another conventional magnetron sputtering apparatus in a round shape and its principal structural members are the same as that shown in FIG. 13.
The conventional magnetron sputtering apparatuses as shown in FIGS. 13 and 14 are designed to trap electrons that are components of the plasma in parallel to the target 1 among a leakage magnetic field from the inner and outer magnetic poles 2 and 3 and that scatter from the target 1, thus facilitating ionization of gaseous molecules and permitting sputtering at a high speed.
In these conventional magnetron sputtering apparatuses, the trapped electrons near the surface of the target 1 are enclosed within a tunnel-shaped magnetic field, as shown by the arrows in FIGS. 13 and 14, and they are allowed to move along the tunnel. Accordingly, it is to be understood that the degree of sputtering of the surface of the target 1 depends upon the distribution of the vertical and horizontal components of the magnetic field with respect to the upper surface of the target 1. FIGS. 15(a ) and 15(b) show distributions of the horinzontal and vertical components, respectively, of the magnetic field above the upper surface of the target 1 as a function of distance from the center of the target 1 to an edge thereof (as shown by line A--B in FIG. 13), the target 1 being made of a ferromagnetic material such as iron or cobalt.
FIG. 16 shows the pattern of erosion of the target 1 shown in the cross section taken between the points A and B in FIG. 13. Erosion is shown to develop locally, thereby shortening the life of the target 1 to a considerable extent.
In order to overcome the problem of localized erosion, a number of longitudinal grooves have been provided on the surface of the target 1 to allow a horizontal magnetic field to be created over the surface of the target (GT Target: Nippon Kinzoku Gakkaiho, Vol. 25, No. 6, P562 (1986)). This solution suffers from the disadvantage, however, that it is not easy to prepare the target 1.
It has been found that the location where erosion develops corresponds to the position where the polarity of the vertical components of the magnetic field changes, namely, at a point where the vertical components thereof become zero, without any direct relationship to the intensity of the horizontal components of the magnetic field.
This finding is based on the assumption that the concentration of the trapped electrons becomes higher near the point where the vertical components become zero, while as shown in FIGS. 13 and 14, the electrons move periodically in a direction in which a gradient of the vertical components of the magnetic field is present, as well as along the magnetic field tunnel on the surface of the target 1. Once erosion occurs on the target 1, the leakage magnetic field, as shown in FIG. 15, becomes larger and the gradient of the vertical magnetic field at the position of erosion gets larger, thus further increasing local erosion.
A conventional large-scale magnetron sputtering apparatus primarily uses an electromagnet for its inner and outer magnetic poles; however, the electromagnet has a higher operating cost. Accordingly, the use of a permanent magnet is desired, but the flux density of the outer magnetic pole must be made larger because the flux density of the outer magnetic pole is reduced around the exterior circumference of the outer magnetic pole compared with the inside thereof.